Network Working Group G. Huston
Request for Comments: 4692 APNIC
Category: Informational October 2006
Considerations on the IPv6 Host Density Metric
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This memo provides an analysis of the Host Density metric as it is
currently used to guide registry allocations of IPv6 unicast address
blocks. This document contrasts the address efficiency as currently
adopted in the allocation of IPv4 network addresses and that used by
the IPv6 protocol. Note that for large allocations there are very
significant variations in the target efficiency metric between the
two approaches.
Table of Contents
1. Introduction ....................................................2
2. IPv6 Address Structure ..........................................2
3. The Host Density Ratio ..........................................3
4. The Role of an Address Efficiency Metric ........................4
5. Network Structure and Address Efficiency Metric .................6
6. Varying the HD-Ratio ............................................7
6.1. Simulation Results .........................................8
7. Considerations .................................................10
8. Security Considerations ........................................11
9. Acknowledgements ...............................................11
10. References ....................................................12
10.1. Normative References .....................................12
10.2. Informative References ...................................12
Appendix A. Comparison Tables ....................................13
Huston Informational [Page 1]RFC 4692 IPv6 Host Density Metric October 20061. Introduction
Metrics of address assignment efficiency are used in the context of
the Regional Internet Registries' (RIRs') address allocation
function. Through the use of a common address assignment efficiency
metric, individual networks can be compared to a threshold value in
an objective fashion. The common use of this metric is to form part
of the supporting material for an address allocation request,
demonstrating that the network has met or exceeded the threshold
address efficiency value, and it forms part of the supportive
material relating to the justification of the allocation of a further
address block.
Public and private IP networks have significant differences in
purpose, structure, size, and technology. Attempting to impose a
single efficiency metric across this very diverse environment is a
challenging task. Any address assignment efficiency threshold value
has to represent a balance between stating an achievable outcome for
any competently designed and operated service platform while without
setting a level of consumption of address resources that imperils the
protocol's longer term viability through consequent address scarcity.
There are a number of views relating to address assignment
efficiency, both in terms of theoretic analyses of assignment
efficiency and in terms of practical targets that are part of current
address assignment practices in today's Internet.
This document contrasts the address efficiency metric and threshold
value as currently adopted in the allocation of IPv4 network
addresses and the framework used by the address allocation process
for the IPv6 protocol.
2. IPv6 Address Structure
Before looking at address allocation efficiency metrics, it is
appropriate to summarize the address structure for IPv6 global
unicast addresses.
The general format for IPv6 global unicast addresses is defined in
[RFC4291] as follows (Figure 1).
| 64 - m bits | m bits | 64 bits |
+------------------------+-----------+----------------------------+
| global routing prefix | subnet ID | interface ID |
+------------------------+-----------+----------------------------+
IPv6 Address Structure
Figure 1
Huston Informational [Page 2]RFC 4692 IPv6 Host Density Metric October 2006
Within the current policy framework for allocation of IPv6 addresses
in the context of the public Internet, the value for 'm' in the
figure above, referring to the subnet ID, is commonly a 16-bit field.
Therefore, the end-site global routing prefix is 48 bits in length,